In the Tupolev 154 crash, it turns out that the problem was not with the aircraft at all, but with the pilot, who broke the rules of instrument navigation while on final approach to the airport.

In the aftermath of the Tupolev 154 crash which killed Poland's president last Saturday, scrutiny initially focused on the aircraft, an obsolescent design that hard largely been retired from fleets elsewhere in the world. The implication was that here in the West, where aircraft are newer and better maintained, this accident would never have happened. But it turns out that the problem was not with the aircraft at all, but with the pilot, who broke the rules of instrument navigation while on final approach to the airport. This violation, called "busting minimums," is a common form of pilot error, which makes up three-quarters of all general aviation fatalities (pdf) in the United States.

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The airfield at Smolensk is equipped with a fairly low-tech kind of navigational system called a non-precision approach. Unlike more sophisticated precision approaches in use at major U.S. airports, non-precision approaches do not provide pilots with information about their vertical distance from the flight path. To stay a safe height above the ground, aircrew descending toward the airport must keep a careful eye on their altimeters. They must not descend below a certain height—the minimum descent altitude (MDA)—without first catching sight of the runway below. If they reach that altitude and the runway is still obscured by cloud or fog, they must initiate a "missed approach" by adding power and climbing away from the airport. At this point they can either try again or fly to another airport where weather conditions are more favorable.

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Sometimes, however, a pilot is highly motivated to get on the ground, a state of mind known colloquially in aviation circles as get-there-itis. He might be suffering mechanical problems, a fuel shortage or simply be impatient to get where he's going. Instead of abandoning his approach, he continues lower, hoping that by pressing on a little longer he'll emerge from the clouds, spot the runway and accomplish his landing. He might figure that, since there's a certain amount of safety margin built into the MDA, there's no harm in pushing it a little bit. But busting minimums, as this behavior is called, can be an insidiously dangerous pastime.

As far as safety margins go, a little bit quickly becomes a little more, and then a lot. Meanwhile, the ground grows closer, time grows short and the pilot becomes stressed out and overloaded, as he tries to simultaneously keep track of various parameters such as altitude, airspeed, direction and location. Psychologically, the intense pressure only makes matters worse, as high levels of fight-or-flight hormones shrink the working memory and hamper rational thought. By the time trees and buildings emerge from the haze, it may be too late.

In the case of the Smolensk incident, the pilot had already attempted the approach three times, and each time failed to identify the runway through the fog. But his passengers, an elite delegation of high-ranking Polish politicians, were eager to get on the ground so that they could attend an important ceremony with their Russian counterparts. This was a group that was used to getting what it wanted. In the past, the Polish president had even threatened to punish a pilot who refused to land in bad weather. The pressure was on. Get-there-itis set in, and the stage was set for tragedy.

Could technology have prevented this? Certainly. Modern aircraft such as the Boeing 777 can all but land themselves at well-equipped airports even in zero visibility. But there are tens of thousands of airports around the world too small to justify the expense of such sophisticated gear, and hundreds of thousands of airplanes with only the most rudimentary navigational equipment. Responsibility for flying them safely is going to remain in the hands of the individual pilots.

The lesson to take from the crash is one that applies in many spheres of human activity. When planning an activity that might involve acute physical danger, set concrete limits beforehand, and stick to them. Climbers ascending the last leg up Everest need to set a firm turnaround time by which they must begin their descent, even if they're within spitting distance of the summit. Scuba divers at the limits of their depth capabilities must respect their dive tables. A small step beyond the limit may seem harmless, but it opens the door to much larger deviations that can prove fatal.

Jeff Wise is a contributing editor to Popular Mechanics and the author ofExtreme Fear. For a daily dose of news on the science of fear, follow his blog here.